Theory of hydrogen solubility in binary iron alloys based on ab initio calculation results

Interaction of hydrogen atoms with three substitutional impurities (X?=?Pd, Ti, Cr) in bcc iron base solid solution was modelled ab initio using the WIEN2k package. It was shown that in spite of attraction between H and X atoms, excess energy of the H atom in tetrahedral sites in the first sphere of coordination of the X atom has a significant positive value, while the lowest negative values are observed in the second (Pd, ?0.087?eV; Ti, ?0.091?eV) or the third (Cr, ?0.032?eV) sphere. A new thermodynamic theory of hydrogen solubility in dilute bcc Fe–X alloys was developed on the basis of these results. The resulting equation was used to analyze existing experimental data on H solubility in a number of Fe–X alloys, and X–H interaction energies were determined for each case. The energies determined from high-temperature solubility data for Fe–Pd, Fe–Ti and Fe–Cr are somewhat greater than those obtained in ab initio calculations. The theory gives a new basis for analyzing hydrogen behaviour in iron-base solid solutions.